An important part of modern building design is the protection of structural steel such as columns, beams, girders and other steel assemblies from the damaging effect of fire. Steel does not burn, but can lose strength at high temperatures. As a result, a variety of fire protection systems, namely mineral insulants, cementitious sprays and intumescent coatings, have been developed to insulate steel from the effects of fire in order to prolong the time required for steel to reach a temperature of about 538° C., generally by one to two hours, depending upon local fire regulations.
Intumescent coatings are coatings that react under the influence of heat and swell to 10-100 times their original thickness, producing an insulating char that protects the substrate to which the coating is applied from the effects of fire. Because intumescent coatings are needed at a relatively low thickness, as compared with the thickness required for other types of insulating materials to achieve a similar fire protection rating, they are increasingly becoming the preferred choice for structural fire protection. Another attractive feature of intumescent coatings is their smooth and aesthetically pleasing finish. Thin film intumescent coatings therefore allow architects and designers to maximize the creative design possibilities of structural steel.
Typical intumescent coatings usually comprise a minimum of four components: a source of mineral acid catalyst, typically ammonium polyphosphate; a source of carbon, typically pentaerythritol or dipentaerythritol; a blowing agent, typically melamine; and a binder, typically a thermoplastic resin. When an intumescent coating is subjected to heat, a series of reactions occur. The ammonium polyphosphate decomposes to produce polyphosphoric acid, catalyzing the dehydration of pentaerythritol to produce char. The blowing agent also starts to decompose, giving off non-flammable gases that cause the carbon char to foam, thus producing a meringue-like structure that is highly effective in insulating the substrate from heat. The basic function of the binder is to bind together the components of the intumescent coating, so that they may be applied to the substrate and held in intimate contact therewith until required to perform their function in a fire situation. Furthermore, the binder contributes to the formation of a uniform cellular foam structure, since the molten binder helps trap the gases given off by the decomposing blowing agents, thus ensuring a controlled expansion of the char. Binder content must be controlled at certain level. Insufficient binder will result in coating defects such as cracking or delamination, whereas too much organic binder will weaken the char strength and cause higher flame spread rate.
Intumescent coatings are generally categorized into three types: water-based (W/B), solvent-based (S/B), and epoxy-based (E/B). Water-based and solvent-based intumescent coatings are among the most widely used products (over 80% usage in the North American market). These coatings utilize a thermoplastic binder, such as polyvinyl chloride (PVC), polyurethane, polyester, polyvinyl acetate, phenolic resin, acrylic resin, and copolymers of ethylene vinyl acetate, or styrene acrylics. The thermoplastic characteristics of the binder allow the coating to swell significantly (with blowing agent) and form chars 10-100 times the original coating thickness. Therefore, only a relatively thin film is required with water-based or solvent-based coatings. However, significant drawbacks of these types of coatings exist in application and handling.
In application, depending on the coating thickness required for fireproofing, a steel member may require 2 to 10 layer of coats. Since only a limited thickness (usually 40-50 mils or 1.0-1.2 mm per day) can be sprayed in a single application without sagging or peeling, and the coating must be allowed to dry (greatly affected by environmental conditions, such as humidity and temperature) before a second layer can be applied, a project may take up weeks or months to finish. Additionally, a trained applicator must apply the coating to ensure that a uniform thickness is applied. For solvent-based systems, the applicator must be aware of special safety considerations, for example inhalation hazards and flammability. Finally, sprayed on coatings are messy and necessitate extensive cleanup of the job site following installation.
In shipping and handling, water-based intumescent coatings quality deteriorates when subjected to freezing or hot temperatures (above 35° C.) conditions. Due to the instability of certain ingredient in water, water-based intumescent coatings usually have shelf lives of about 6 months.
Epoxy-based coatings (e.g. PPG's Pitt-Char® and Akzo Nobel's Chartek® systems) have great durability and are mostly used for outdoor applications, such as offshore platforms or industrial plants. Because of the thermosetting nature of epoxy resins, epoxy-based coatings swell poorly upon heating (only a few times their original thickness) and consequently require greater amounts to be applied in order to attain the desired fire protection rating. The cost of epoxy systems is usually much higher than water-based and solvent-based systems, meaning that the overall project cost is prohibitive for interior applications. In addition, the aesthetic finish is compromised due to the much greater coating thickness required.
Coatings are often reinforced using, for example, short length pieces of glass fibers or mineral fibers mixed with the coating during application. The random direction of the fibers mixed throughout the coating lends reinforcement, reducing the likelihood of sagging, and allowing greater overall coating thickness to be applied to increase fire protection ratings beyond what can be achieved without reinforcement. These fibers are also well known to enhance char strength during fire. However, incorporating long fibrous materials in intumescent coating is prohibitive due to spraying difficulties and aesthetic concerns.
100% solid intumescent fire proofing materials are described extensively in the literature. US 2008/02241005 A1 describes a liquid curable intumescent system by radical polymerization. US 2012/0174508 A1 describes curable intumescent system using a silane-terminated polyurethane or a silane terminated polyether as binder. U.S. Pat. No. 7,217,753 B2 describes a powder coating system using phenolic resin capable of protecting steel substrate for 2 hours. All these systems are thermosetting, potentially requiring sophisticated installation equipment.
Ammonium polyphosphate, melamine, polyalcohol are well described in literature as non-halogenated fire retarding compound for thermoplastic materials. U.S. Pat. No. 6,043,304 describes a flame-retarded hot-melt adhesive composition, which may be applicable to dimensionally heat-recoverable articles to flow and seal when the articles are heat-recovered in use. U.S. Pat. No. 6,809,129 B2 describes a moldable elastomeric intumescent material comprises chlorinated polyethylene, plasticizers, phosphate based foaming agents, char forming materials, antioxidants, intumescent materials, flame retardant materials, and graphite and/or expandable graphite. JP 2002/322374 describes a moldable intumescent sheet composition for fire protection, which is typically flexible or non-rigid, having an organic binder content typically above 30 wt % of total dry composition and an expansion ratio of up to about 10.
There is a need in the art for improved intumescent materials for fireproofing, which may overcome one or more of the problems associated with current fire resistant materials, for example: time associated with the application of the required thickness of conventional coatings; extended drying time associated with conventional coatings (mostly W/B and S/B); limited installation conditions; damaged coating difficult to repair; short shelf life of water-based coating; necessity for jobsite installation; large thickness associated with E/B; expensive equipment required for installation; and extensive cleaning of job site.